Monocular borescope or endoscope with offset mask

ABSTRACT

A monocular borescope or endoscope is provided with an image relaying means ( 16 ) defining a longitudinal axis (L)(and with a viewing means comprising an ocular lens ( 20 ) and field mask ( 18 ) defining a central axis (C) which is parallel to and offset from the longitudinal axis (L). In one embodiment the ocular lens ( 20 ) is coincident with the central axis (C) of the mask ( 18 ), while in another embodiment the ocular lens ( 20 ) is coincident with the longitudinal axis (L). The mask ( 18 ) is arranged to be mountable in a number of different rotational orientations each providing a different offset between the longitudinal axis (L) and the central axis (C).

The present invention relates to rigid monocular borescopes andendoscopes which are well known devices for viewing objects at remote orinaccessible locations. The invention in particular relates to means forproviding an accurate direction of view in such rigid borescopes andendoscopes.

Although there are many detailed design variations, rigid borescopes andendoscopes generally consist of the basic arrangement shownschematically in FIG. 1. When an object O lies within the scope's fieldof view (f.o.v.), an image I of the object O is formed within theinsertion tube 12 of the scope 10 by means of an objective lens assembly14. This intermediate image I is then transferred or relayed to theproximal end of the scope 10 by a series of relay lenses 16 to form afinal intermediate image superimposed on a field mask 18. The purpose ofthe field mask 18 is to provide a sharp, well-defined edge to thespecified field of view. Finally an ocular lens system 20 then projectsthis final image to form a virtual image some distance away, say about 1m, so as to present a comfortable view to the eye E (a normal, unaidedeye can generally focus comfortably on objects from a distance of about25 cm to infinity). Additionally an adaptor lens attachment (not shown)may be fitted to re-focus this image on to a camera or CCD chip toprovide an image of appropriate size for the camera.

The distal end of the scope 10 will generally use a prism arrangement 22to establish the required direction of view (d.o.v.), unless this is a0° forward viewer in which case no prism is required. The direction ofview is the angle between the longitudinal axis L of the scope 10 andthe viewing axis V where the viewing axis is defined as the linebisecting the extreme directions delineating the diametric edges of thevisible field of view. The actual direction of view that is achieved inpractice may vary from the design specification for a number of reasons:

the prism 22 may not be set at precisely the correct angle

the individual lenses 14, 16 comprising the objective and relay systemsmay not be manufactured such that the optical and mechanical centrescoincide precisely

the individual lenses 14, 16 comprising the objective and relay systemsmay not be precisely centred or aligned in the lens tube 12

In a borescope or endoscope there may be large numbers of individuallenses and the build up of very slight centring errors of this type mayeasily result in a shift in the measured d.o.v. outside acceptablelimits. The conventional method of controlling this is to place verytight tolerances on the items listed above in order to ensure theresultant d.o.v. is within the specified limits (typically ±5°). Forscopes where the specified tolerance for d.o.v. is particularly tight(e.g. ≦3°) it may not be possible (or would be prohibitively expensive)to control these individual tolerances to the required level ofprecision. There is therefore a need to achieve a high tolerance d.o.v.without relying on particularly tight individual centration tolerances.

Accordingly, the present invention provides apparatus for use as amonocular borescope or monocular endoscope, comprising a tube having adistal end and a proximal end connected to a housing, a viewing portadjacent a distal end through which an object may be viewed in use, animage relaying means operable to relay an image of the object to aviewing means provided in the housing, the viewing means including anocular lens and a mask positioned distally of the ocular lens and havingan aperture through which the image is viewed, wherein the imagerelaying means defines a longitudinal axis and the aperture of the maskdefines a central axis and wherein the central axis is parallel to andoffset from the longitudinal axis.

In a first embodiment, the ocular lens defines a central axis which iscoincident with the longitudinal axis.

In a second embodiment, the ocular lens defines a central axis which iscoincident with the central axis of the aperture in the mask.

In order to provide the desired offset, the mask may be mounted so as tobe adjustable into the correct position. Alternatively, the mask may bemoved and replaced with a second mask having the correct offset.However, the current preference is for the mask to be mountable indifferent rotational orientations each providing a different offsetbetween the longitudinal axis of the image relaying means and thecentral axis of the mask. Preferably the ocular lens is adjustabletogether with the mask. This allows the mask to be provided withdifferent offsets, but does not require a selection of different masksto achieve this aim.

The present invention also provides a method of providing a desireddirection of view in a monocular borescope or monocular endoscope, theborescope or endoscope comprising a tube having a distal end and aproximal end connected to a housing, a viewing port adjacent to thedistal end through which an object may be viewed in use, an imagerelaying means operable to relay an image of the object to a viewingmeans provided in the housing, and the viewing means comprising anocular lens; the method comprising the steps of providing a maskdistally of the ocular lens, the mask having an aperture through whichthe image is viewed and the aperture having a central axis; determiningthe angular difference between the actual direction of view and thedesired direction of view; calculating the amount and direction ofoffset of the central axis of the mask aperture relative to thelongitudinal axis required to achieve the desired direction of view anddetaching the mask and fitting a mask having an aperture defining acentral axis with the required offset.

The steps of detaching and fitting the mask preferably comprisedetaching the mask, rotating the mask to a different rotationalorientation with the required offset and fitting the mask into position.Preferably, the ocular lens is rotated together with the mask to therequired offset.

The method may further comprise step of replacing the ocular lens with asecond lens having a central axis coincident with the central axis ofthe second mask.

In the method, the amount of offset required may be calculated inaccordance with the relationship:$\delta = \frac{\alpha \cdot \eta}{\phi}$

where δ is the amount of offset required, α is the angular differencebetween the actual direction of view and the desired direction of view,η is the size of the image at the position of the mask and φ is thefield of view of the scope.

The invention will now be described in detail, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of a prior art rigid borescope or endoscope;

FIG. 2 is a schematic diagram illustrating a first embodiment of thepresent invention;

FIG. 3 is a schematic view illustrating a second embodiment of thepresent invention;

FIG. 4 is a cross section through the proximal end of a borescope withthe handle removed for clarity; and

FIG. 5 is a perspective view of the borescope of FIG. 4 with the housingremoved.

FIG. 1 shows in schematic form the configuration of a conventional rigidborescope or endoscope and has been discussed above.

As mentioned above the scope 10 has a longitudinal axis L. The fieldmask 18 is a plate with a circular aperture, the aperture having acentral axis C passing through its centre. In the prior art, the centralaxis C is coincident with the longitudinal axis L. An orbital scanningscope will also have a rotational axis R which is usually coincidentwith the longitudinal axis L.

A first embodiment of the present invention, for incorporation in ascope of the type shown in FIG. 1, is illustrated schematically in FIG.2.

In the present invention, the field mask 18′ is shifted laterallycompared with a conventional field mask 18 (shown for comparisonpurposes only) so that its central axis C remains parallel to but is nolonger coincident with the longitudinal axis L of the scope as a whole.Accordingly, the virtual image 30′ of the field mask 18′ formed by theocular lens, and thus the image 30 on the retina of the eye E (or in thecamera), are offset laterally from the virtual image 32′ of the objectand the corresponding image 32 on the retina, respectively. In turn,this shifts the measured direction of view of the scope by a smallamount.

In the first embodiment illustrated in FIG. 2, the field mask 18′ ispositioned in a housing (not shown) at the proximal end of the insertiontube (not shown) such that the central axis C of the aperture will beoffset from the longitudinal axis L of the scope 10. This embodiment isentirely suitable for correcting the direction of view in scopes inwhich there is no orbital scan facility, i.e scopes in which theinsertion tube is not rotated about its longitudinal axis.

However, this embodiment is not suitable where orbital scan is requiredor where the scope 10 is rotated relative to the eye or attached camera.This is because, in offsetting the field mask 18′ from the longitudinalaxis L of the scope, an offset in the position of the image 30 of thefield mask 18′ formed on the retina of the eye E (or in the camera etc.)relative to this axis L, and relative to the image 32 of the object onthe retina, is also introduced. If the optical system now rotates aboutits longitudinal axis L the direction of the bundle of rays associatedwith the centre of the field mask will precess around the longitudinalaxis L causing the field mask image position as formed on the retina ofthe eye or on the CCD chip of a video camera also to move in rotarymotion about the longitudinal axis L. This effect will be noticeablewhen viewing with the naked eye but would be particularly apparent whenviewing by means of an attached video camera and could even cause thefield mask image to shift off the active area of the CCD chip as thescope rotates.

In order to overcome this problem, a second embodiment of the presentinvention is provided and is illustrated in FIG. 3.

In this second embodiment, both the field mask 18′ and ocular lens 20′of the scope 10 are offset from the longitudinal axis L of the scope 10by the same amount and in the same direction. (A conventional field mask18 and ocular lens 20 are also shown for comparison purposes only.) Thisresults in a shift in the measured direction of view of the scope 10.However, as the tube 12 is rotated about the longitudinal axis L,although the bundle of light rays associated with the centre of thefield mask will shift in position laterally about the longitudinal axisL, their direction does not change and since the ray bundle issubstantially parallel, i.e the eye or camera lens is essentiallyfocussed at infinity, this does not give rise to any shift in theposition of the field mask image 30 on the retina of the eye or the CCDcamera chip as the scope rotates. Accordingly, this embodiment issuitable for orbital scan scopes without resulting in any field maskimage movement.

The present invention can be implemented in various ways which are easyto implement in the final stages of scope assembly.

Typically, the actual direction of view of a scope is measured using astandard field mask and ocular assembly with a central axis C coincidentwith the longitudinal axis L of the scope so as to determine the amountand direction of field mask shift which will be required to give adesired direction of view.

This desired d.o.v may then be provided by selecting an appropriateocular assembly from a range of assemblies constructed to provideincrementally differing amounts of offset in the position of the fieldmask 18′ and ocular lens 20′. Alternatively, the ocular assembly foreach particular scope may be machined such that the internal bore isoffset by precisely the amount calculated to achieve the desireddirection of view.

As those skilled in the art will appreciate, there is a limit to theamount of direction of view correction which is achievable by thismeans. If the shift in the field mask position is too great then theimage seen within the field mask will start to be clipped by thephysical boundaries of the lens tube, prism, relay or objective lensesand so on. The present invention therefore relaxes the centrationtolerances on a borescope assembly to within practical limits eventhough it does not remove them entirely.

The level of offset required in order to provide a given shift in d.o.v.will depend on the particular design of the borescope. In general we canstate the following relationship between the required d.o.v. shift α,the field of view φ, the intermediate image size (at the field maskposition) η and the amount of offset required δ:$\delta = \frac{\alpha \cdot \eta}{\phi}$

The intermediate image size for a scope is a design parameter selectedagainst a variety of other parameters to achieve particular performancetrade-offs. However, it will generally be some fraction ε of the scopediameter D. Hence the above equation becomes:$\delta = \frac{\alpha \cdot ɛ \cdot D}{\phi}$

This equation is only strictly accurate for a system with zerodistortion. However, it serves to indicate the general relationships. Itwill be noted that to achieve a given shift in d.o.v. the ocular offsetrequired increases with the diameter of the scope, and inversely as thefield of view. Typically, in order to achieve a 1° shift in d.o.v. a 6mm diameter scope covering a 60 degree field of view would require anoffset of 0.05 mm. A 10 mm diameter scope covering a 35° field of viewwould require an ocular offset of 0.19 mm.

FIGS. 4 and 5 show a borescope in accordance with the second embodimentin which a single mask assembly can be used to provide a number ofdifferent offsets.

Many of the features of the borescope of FIGS. 4 and 5 are conventionaland are only described briefly below.

The housing 50 is provided at its distal end with an orbital scancontrol 51 while a lens tube sleeve 52 extends from the housing andsupports a lens system 52′ extending to the distal end of the borescope.The viewing means comprises a housing eye piece 53 have a window 54 anda conventional focus control 56 which advances a ball 57 in a helicalgroove 57′ to alter the axial position of ocular body assembly 58.

The ocular body assembly 58 comprises the field mask 18′ at its distalend which is adjusted to the correct axial position by means ofpacking/spacer shims, while a dove prism 59 may optionally be providedat its distal end. The ocular body assembly 58 has four axiallyextending grooves 60 which equally spaced about its circumference. Anorbital scan link pin 61 extends from the lens tube sleeve 52 into oneof the grooves 60 to ensure that the lens tube 52 rotates together withthe ocular body assembly 58.

As best shown in FIG. 5, the mask 18′ is located within the ocular bodyassembly by means of an alignment pin 62. The mask is provided with fourgrooves 63 (only two of which are shown in FIG. 5) any one of which canbe engaged with the alignment pin 62. A field mask direction of viewindicator 64 indicates the orientation of the field mask 18′ within theocular body assembly 58. The field mask 18′ is retained in the ocularbody assembly 58 by a locking ring (not shown) which engages with theouter peripheral area of the field mask 18′.

When the borescope is being assembled, the degree of offset required ofthe mask is determined in accordance with the equation given above. Thelens tube sleeve 52 is then mounted to the ocular body assembly to givea direction of view in accordance with acceptable limits. Thus, if theoffset is determined to be outside of the acceptable limits, the ocularbody assembly is rotated to an orientation in which it gives a positiveor negative shift in the direction of view. If the offset is withinacceptable limits, the ocular body assembly is mounted in a nominalposition that just provides a shift in direction of view in a directionnormal to the plane containing the longitudinal slope axis and thenominal viewing axis.

Depending upon the relative positions of the lens tube sleeve 52 andocular body assembly 58, it may also be necessary to change theorientation of the field mask 18′ so that the direction of viewindicator 63 points in the right direction. It should be noted that asthe field mask is mounted so as to be concentric with the ocular lens20′, this rotation of the field mask 18′ does not provide any additionaloffset to the image, but simply changes the position of the direction ofview indicator 64.

What is claimed is:
 1. Apparatus for use as a monocular borescope ormonocular endoscope, comprising a tube having a distal end and aproximal end connected to a housing, a viewing port adjacent a distalend through which an object may be viewed in use, an image relayingmeans operable to relay an image of the object to a viewing meansprovided in the housing, the viewing means including an ocular lens anda mask positioned distally of the ocular lens and having an aperturethrough which the image is viewed, wherein the image relaying meansdefines a longitudinal axis and the aperture of the mask defines acentral axis and wherein the central axis is parallel to and offset fromthe longitudinal axis.
 2. Apparatus as claimed in claim 1, wherein theocular lens defines a central axis which is coincident with the centralaxis of the aperture in the mask.
 3. Apparatus as claimed in claim 1,wherein the ocular lens defines a central axis which is coincident withthe longitudinal axis.
 4. Apparatus as claimed in claim 1, wherein themask is mountable in different rotational orientations each providing adifferent offset between the longitudinal axis of the image relayingmeans and the central axis of the mask.
 5. Apparatus according to claim4, wherein the ocular lens is rotatable together with the mask betweenthe different rotational orientations.
 6. A method of providing adesired direction of view in a monocular borescope or monocularendoscope, the borescope or endoscope comprising a tube having a distalend and a proximal end connected to a housing, a viewing port adjacentto the distal end through which an object may be viewed in use, an imagerelaying means operable to relay an image of the object to a viewingmeans provided in the housing, and the viewing means comprising anocular lens; the method comprising the steps of providing a maskdistally of the ocular lens, the mask having an aperture through whichthe image is viewed and the aperture having a central axis; determiningthe angular difference between the actual direction of view and thedesired direction of view; calculating the amount and direction ofoffset of the central axis of the mask aperture relative to thelongitudinal axis required to achieve the desired direction of view anddetaching the mask and fitting a mask having an aperture defining acentral axis with the required offset.
 7. A method according to claim 6,wherein the step of detaching and fitting the mask comprises detachingthe mask, rotating the mask to a different rotational orientation withthe required offset and fitting the mask into position.
 8. A methodaccording to claim 7, wherein the ocular lens and mask are coaxial, andthe method further comprises rotating the ocular lens to the requiredoffset together with the mask.
 9. A method as claimed in claim 6,further comprising the step of replacing the ocular lens with a secondlens having a central axis coincident with the central axis of thesecond mask.
 10. A method as claimed in claim 6, wherein the mask has aprojection providing a visual indication of its orientation, and themethod further comprises rotating the mask on its own about its axis tochange the position of the projection.
 11. A method as claimed in claim6, further comprising calculating the amount of offset required inaccordance with the relationship;$\delta = \frac{\alpha \cdot \eta}{\phi}$

where δ is the amount of offset required, α is the angular differencebetween the actual direction of view and the desired direction of view,η is the size of the image at the position of the mask and φ is thefield of view of the scope; and positioning the mask at the calculatedoffset.